Considerable efforts have been directed toward the research and development of light-emitting devices including semiconductor light-emitting elements in combination with phosphors. Such light-emitting devices are gaining attention as next-generation light-emitting devices having low power consumption, reduced size, and high luminance and capable of reproducing a wide range of colors. Semiconductor light-emitting elements typically emit primary light in the near-ultraviolet to blue region, for example, at wavelengths of 380 to 480 nm. Also proposed are light-emitting devices including various phosphors suited for different applications.
For example, such light-emitting devices are used as backlights for compact liquid crystal displays (LCDs). An example light-emitting device in this field is disclosed in Japanese Unexamined Patent Application Publication No. 2003-121838 (PTL 1). PTL 1 discloses a backlight light source having its spectral peak in the range of 505 to 535 nm. This backlight light source includes a green phosphor activated with europium, tungsten, tin, antimony, or manganese. Specifically, MgGa2O4:Mn and Zn2SiO2:Mn green phosphors are disclosed in the Examples in PTL 1. Unfortunately, these green phosphors exhibit noticeably low emission efficiency when excited by light having a peak wavelength of 430 to 480 nm (the primary light discussed above).
An example red phosphor is a tetravalent-manganese-activated tetravalent metal fluoride phosphor disclosed in U.S. Patent Application Publication No. 2006/0169998 (PTL 2). In PTL 2, however, there is no discussion of a light-emitting device in which semiconductor light-emitting elements are used in combination with phosphors in order to improve output performance.
Various semiconductor light-emitting devices for applications such as lighting systems have also been developed, and various approaches to improve output performance have been researched. For light-emitting devices that can be used as general lighting fixtures, good color rendering properties are an important performance factor along with improved output performance. (The term “good color rendering properties” basically means a general color rendering index Ra of 80 or more. Color rendering properties can be determined, for example, in accordance with the U.S. Energy Star Standards.) For example, Japanese Unexamined Patent Application Publication No. 2012-124356 (PTL 3) discloses a chip-on-board (COB) light-emitting device manufactured by directly mounting a plurality of light-emitting diode (LED) chips on a board, connecting the LED chips to a circuit via wiring lines on the board, and sealing the LED chips with resin. PTL 3 teaches that good color rendering properties can be achieved using two red phosphors of different peak wavelengths.
Unfortunately, a light-emitting device including only a (Sr1-y,Cay)1-xAlSiN3:Eux phosphor (hereinafter referred to as “(Sr,Ca)AlSiN3:Eu phosphor”) and/or Ca1-xAlSiN3:Eux (hereinafter referred to as “CaAlSiN3:Eu phosphor”), which are used in PTL 3, may exhibit low luminous efficiency (in units of lm/W) relative to the power supplied thereto since most light components fall within a long-wavelength region of 700 nm or more (where the relative sensitivity of the human eye is low). The phosphors used in PTL 3, i.e., (Sr,Ca)AlSiN3:Eu phosphors and CaAlSiN3:Eu phosphors, also absorb secondary light emitted from green phosphors since they show light absorption at wavelengths of 500 to 600 nm, mainly in the green region. This results in two-step light emission in which the red phosphors emit light by absorbing secondary light emitted from green phosphors, thus leading to decreased emission efficiency.
The relative sensitivity of the eye is internationally defined by the relative sensitivity curve of the CIE (International Commission on Illumination) standard observer and is also defined in Japan as the spectral luminous efficiency in Appended Table 8 of Rules for Units of Measurement in Ordinance No. 189 of the Ministry of Economy, Trade and Industry. According to these standards, the human eye is most sensitive to light around a wavelength of 555 nm in bright light and is most sensitive to light around a wavelength of 507 nm in low light. The human eye is less sensitive to light at a wavelength longer or shorter than the peak wavelength and thus senses the light to be dimmer. The luminous efficiency for red light is lower at a longer wavelength.